When color images are printed onto recording sheets by an inkjet method, “bleeding” tends to occur at a color interface where two colors are overlapping.
In order to mitigate bleeding, a surface-active agent may be added to the ink to enhance the permeability of ink into paper sheets (see, for example, patent document 1).
By enhancing the permeability of ink into paper sheets, the bleeding at color interfaces can be mitigated; however, “feathering” tends to occur. Specifically, the ink permeates along the cellulose fiber forming the recording sheet, and therefore feathering occurs around the printed characters and fine lines.
In order to prevent both bleeding and feathering, there is a method of using ink having low permeability only for printing black characters. However, the black ink has poor drying properties, and therefore it is difficult to attain high-speed printing with this method.
Patent document 2 proposes a method for mitigating bleeding in images. Specifically, ink including a complex compound including polyvalent metal ions is pH-adjusted so that the polyvalent metal ions are essentially inactivated. Then, on a recording medium, the ink that has been pH-adjusted is mixed with a liquid having pH for activating the polyvalent metal ions. Accordingly, the color materials in the ink become flocculated so that bleeding is mitigated.
Patent document 3 discloses an inkjet recording method as follows. The ink includes polyvalent metal ions that can flocculate anionic resin. The polyvalent metal ions are included as an inactivated metal complex. This ink is discharged onto a coated paper sheet having an ink absorbing layer. After the ink is discharged, the polyvalent metal ions are discharged onto the ink absorbing layer from the metal complex by a means for changing the pH of the ink. According to functions of the discharged polyvalent metal ions, the pigment deposited on the ink absorbing layer is fixed.
According to the methods described in patent document 2 and 3, the thickening properties of the discharged ink can be controlled, and therefore bleeding and feathering in images can be appropriately mitigated. These effects can be achieved without hampering high-speed printing, which would otherwise be caused due to lack of drying properties.
However, with the above methods, it is necessary to prepare a liquid having pH suitable for activating the polyvalent metal ions, in addition to preparing the ink. This requires additional elements provided in the device as well as complex jetting control operations (which lead to reduced reliability).
In patent document 2, the polyvalent metal ions are inactivated with low-molecular anionic compounds. Therefore, the ink may not be sufficiently flocculated for achieving high quality images.
One approach is to provide the recording target material with pH suitable for activating the polyvalent metal ions; however, the recording target material would be limited to special paper, which degrades the freedom in selecting the sheet type by the user.
Patent Document 1: Japanese Laid-Open Patent Application No. S55-65269
Patent Document 2: Japanese Laid-Open Patent Application No. 2005-1259
Patent Document 3: Japanese Laid-Open Patent Application No. 2009-248472
Non-patent Document 1: http://www.chelest.co.jp/products/chelete-products/-nta-2.php
Non-patent Document 2: http://www.kimica.jp/products/NaAlgin/